The disease pendulum is finely balanced

The significant outbreak of mumps in a part of the US, the first in almost 20 years, is a disturbing feature on the infectious disease scene. Over the past five years the average annual number of mumps cases in the US has been 265. By the beginning of April of this year, the US has already seen 365 cases of the disease in the state of Iowa, as well as at least 64 cases in nearby states.

Coming close on the heels of a major epidemic in England and Wales over the last few years, this is a disturbing development. During 2004-05 England and Wales experienced a major mumps epidemic reaching a peak in 2005 when more than 56,000 cases were notified. In total, there have probably been more than 100,000 cases of mumps over the past couple of years.

Most cases were young adults and teenagers, aged between 15 and 24 years, many of whom had missed out on routine mumps vaccination in previous years. Although England and Wales experienced a decline in vaccination rates after 1995, the real reason for the tremendous surge in mumps cases in 2004-05 was because a whole cohort of youngsters missed out on the MMR vaccine in 1994 when a world shortage saw only the measles and rubella vaccines offered to 5-16 year-olds.

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In theory Australians should be well protected against mumps. Australia has a high immunisation rate with more than 91 per cent of children, aged 12-15 months immunised. While registration of mumps in all Australian states does not go back beyond 2000, the number of cases has averaged about 148 per year over the past decade. In 2005 there were 239 cases officially notified, interestingly, almost the same number as in the United States with a population 12 times higher.

Mumps is an acute viral infection characterised by fever and swelling and tenderness of the salivary glands. Probably about 25 per cent of cases do not display any symptoms. Complications can include deafness and meningitis. Before the advent of the MMR vaccine possibly everybody in Australia was exposed to mumps, with 90 per cent of cases occurring in young children and teenagers. Since the advent of the MMR vaccine against measles, mumps and rubella, many thought the days of infections like mumps were numbered.

Most of the recent US cases have been young college students aged between 18 and 23 years. Disturbingly, most of the victims seemed to have received the MMR shot sometime in the past and in theory should have been immune. While the source of the US epidemic remains unknown, a number of critical issues emerge with relevance for Australia.

In the first place, we should realise that immunisation is not a 100 per cent defence against mumps. In about 5 per cent of cases the vaccine does not trigger an immune response and the person remains at risk. Second, it may be possible that the vaccine declines in efficacy in vaccinated people over time. Third, there is the possibility that the current vaccine is less effective than we have been lead to believe. Fourth, it might be possible that there is some connection between the epidemic that raged in England and Wales and the US outbreaks. Finally, it might just be possible that the mumps virus could be developing some sort of resistance to the current vaccine.

Should we be worried? After all, mumps is usually a pretty mild illness in otherwise healthy young people, lasting only about five days. But in an environment in which we have been brought up to believe that vaccination offered complete protection, and that we were moving to a situation were measles, mumps and rubella were infections of the past, it is sobering to realise that the disease pendulum is finely balanced.